Regeneration of exhausted silver bleach solutions by means of n-bromo compounds



Patented Sept. 23, 1952 BLEACH SOLUTIONS BY MEANS OF,

N -BROMO COIVIPOUNDS August H. Brunner, Jr., and Robert H. Zappert,

Binghamton, N. Y., assignors to General Aniline & Film Corporation, New York, N. Y., a

corporation of Delaware No Drawing. Application December 30, 1949, Serial No. 136,146

This invention relates to a photographic process and particularly to the process of regenerating exhausted silver bleaching "solutions.

In the processing of color film it is customary to convert the silver, formed by development, into a hypo-soluble silver salt, such as silver bromide which is subsequently removed by fixing. A similar procedure is involved in removing negative silver in processing black and white reversal film. The most commonly employed silver bleaching solution consists essentially of an aqueous solution of an alkali ferricy'anide and :an alkali bromide, and if desired, small amounts of a buffer, such as an alkali metal phosphate and bisulfate to maintain the hydrogen ion concentration constant.

The bleach solutions, although reasonably stable during storage, lose their 'efiiciency as soon as they have been used for the treatment of relatively small areas of film. During use, the metallic silver image on the film is oxidized to silver ions by the ferricyanide ions which are in turn reduced to ferro'cyanide ions. The silver ions are precipitated by the bromide ions as silver bromide and removed by subsequent treatment of the film in a fixing bath. Since the silver bromide is removed, the net efiect in the bleaching solution is a conversion of ferricyanide ions to ferro'cyani-de ions and a loss of bromide ions. Subsequent bleaching with the used solution not only requires considerably more time but the removal of silver is incomplete, causing dulling of the highlights together with unsightly stain. Replenishment by the customary procedures, such as by the addition of ferricyanide and potassium bromide, which are used up in the bleaching reaction, does not improve the performance of an exhausted bleach solution to any appreciable extent. The analysis of such a solution intended for discard usually indicates that as much as 90 to 95% of the alkali ferricyanide originally added is still present. It becomes manifest that a procedure utilizing only 'a-small fraction of an active ingredient is noto'n ly wasteful but economically unsound, particularly Where as here, the active ingredient, the alkali metal fer'ricy'anide; is relatively expensive.

As pointed out above, the slowing up of the pears to "perform a dual function (1) by 'oXidizing the ferrocyanide ions to ferricyanide ions, and (2) by supplying additional bromide ions. Bro- 12 Claims. (CI. 95-88) mine, itself or its solution in methanol or ethanolor in water, the latter commonly known as bromine water, performs a similar function but tends to destroy some of the ferricyanide. The use of methanol or ethanol as a solvent does not overcome the hazards of handling liquid bromine, and since bromine oxidizes methanol and ethanol readily, the solution is not stable and must be utilized immediately. Moreover, bromine is a powerful skin irritant and even its vapors are toxic, making its use extremelyhazardous. In addition, if accidentally spilled, it may seriously afiect not only the color film, but also tanks,

spools, and similar equipment utilized in film processing.

It is, therefore, an object of the present invention to provide a simple andeconomical method of regenerating an exhausted ferricyanide bleach,

solution.

Other objects and advantages will appear hereinafter.

According to the present invention, exhausted bleach solutions comprising an alkali ferricyanide and an alkali bromide are regenerated or rejuvenated by the action of any organic N- bromo derivatives of a compound containing the groups --CHzNHz, -CONH2, or --(C"O)dNI-I, and derivative of an aromatic compound containing the group, SO2NH2, in which at least one of the hydrogen atoms is substituted by a bromine atom, and including N-bromomelamine and the di-, tri-, and hexa-bromo derivatives of the amides of cyanuric acid, which are capable of releasing elementary bromine in aqueous solutions.

The nature of the aforestated compounds is immaterial so long as they are aliphatic or aromatic in nature or are the mono and polybromo derivatives of the amides of cyanuric acid.

As typical illustrations of such compounds the following may be mentioned:

( 1) CHIIQNIIYBI N-bromobutylamine (2) C4H9NBr2 N-dibromobutylamine (3) C5H-11NB1'2 N-dibromopentylamine (4) CeHisN-EBr N-bromohexylamine (5) C'zHisNBrz N-dibromoheptylamine 6 BI'2N.C2HNB1.C2H4NBI .C2H4NBI'2 N-hexabromotriethylene tetramine (7) CHzCONI-IBI' N-bromoacetaiinide 3 CHsCONBrz N -dibromoacetamide CHaCI-IzCONI-IBr I N -bromopropionamide CHaCHzCONBlz N-dibromoproplonamide N-bromobenzamlde N,'N-dibromo N,N'-diphenylures N -bromophthalimide (EH 0 NHB r N-bromophenylacetamide E10 zNBrNa 7 Sodium N-bromo-p-toluenesulfonamide S 0 NBn N-dlbromo-p-toluenesulfonamide B O INB 1'3 SOzNBIz N-tetrabromobenzene-m-disulfonamide NHB! N-tribromomelamine N -hexa.bromomelamine The use of liquid bromine for rejuvenation of alkali ferrocyanide bleach baths theoretically requires one atom of bromine per molecule of ferrocyanide. When using any one of the aforestated N-bromo compounds, one mol offerrocyanlde requires only one mol divided by twice the number of N-bromo atoms in the compound. By way of example, one mol of ferrocyanide theoretically requires grams of bromine, where as only 69 grams of N -bromo acetamide are theoretically required.

The advantage of utilizin the N-bromo compounds is that they are solids which are reasonably stable for long periods of time, are nonirritating, and produce no toxic vapors. The N- bromo compounds may be added directly to the bleach solution thus avoiding the dilution which occurs with the use of a solvent. Since the reaction involved consumes one equivalent of hydrogen ions, the ions must be replaced by the addition of a small amount of an acid or of an acid salt to the bleach solution, such as sulfuricacid, citric acid, hydrobromic acid, sulfamic acid, or an alkali bisulfate, e. g., sodium bisulfate. A sufiicient amount of the acid or acid salt is added to the exhausted bleach bath so'as to restore the pH to its original value. Solid acids or acid salts may be added directly, but it is advisable to dilute the sulfuric acid with water.

Since bromide ions are consumed in the bleaching process, only half of which are replaced by the N-bromocompound, additional bromide ions may be added'if it is desired to restore the bromide to its original concentration. This can be accomplished by the use of a bromide salt. If hydrobromic acid is used as the acidifying agent, the bromide concentration is automatically restored.

The actual or approximate amount of the N- bromo compound to be added is determined stoiohiometrioally after the determination of the ferrocyanide content by colorimetric, potentiometric, or other suitable titration means. Amounts'of the N-bromo compounds smaller than that the oretically required to oxidize the ierrocyanide ions present in the exhausted bleach bath give somewhat incomplete rejuvenation, whereas any appreciable excess tends to destroy some of the ferrioyanide. A small excess, 1. e., above the theoretical amount, can, however, be tolerated in most instances without impairing the efiiciency of the regenerated bleach solution.

As is well known, many photographic bleach solutions contain an alkali metal ferricyanide,

such as lithium, potassium, or sodium ferricyanide as an oxidizing agent dissolved in a suitable solvent, such as, for example, water with or without the presence of other adjuvants. A typical bleach solution consists of about grams of an oxidizing agent per liter of solution and may contain one or more of the following ingredients: Alkali metal bromide, such as lithium,

potassium and sodium bromide.-." 10 grams (converting agent) Dmlkah metal phosphate 40 grams (bullering agent) Alkali metal bisuliate or orga c acid,

such as citric acid 5 grams (buffering salt) Alkali metal tetraphosphate or hexametaphosphate 20 grams (corrosionandsludgin reventive Water to make 1 liter (solvent). g p

invention.

By an oxidizing agent, we mean any I alkali metal ferricyanide that will oxidize a metallic silver image to form a silver salt. Most commonly'used oxidizing agents are alkali metal ferricyanide, such as lithium, potassium, and sodium ferricyanide or mixture of these salts.

By a converting agent, we mean acompound, such as an alkali metal bromide, which by reason of a diiference in solubility will replace the silver ferrocyanide formed in the bleaching reaction to form silver bromide and thus impel thebleach reaction to completion. Converting agents are sometimes called halogenizing agents.

, By buffering agents, We mean a substance that is added to bring the ferricyanide bleach solution to a specified pH and to maintain .thispI-I even though small amounts of acids or alkali may be carried into the bleach by the film. The pH of the solutions used in bleaching films are maintained between 3 and 9, although apH between 5 and '7 is generally preferred. 1

By corrosion preventive, we-reier to those compounds which retard the corrosion of the ferricyanide bleach on developing. tanks, trays and the like. Large quantities to 80' grams per liter) of alkali metal phosphate, such as disodium phosphate, sodium tetraphosphate, sodium hexametaphosphate, and the like, have been found to prevent corrosion effectively.

By solvent, we refer to such liquid in which the above materials are dissolved and which is usually water.

The following examples describe in detail, methods for accomplishing the above objects, but

it is to be understood that they are inserted merely for the purpose of illustration and are not to be construed as limiting the scope of the Example I l A photographic bleach solution having the following formula:

Potassium ferricyanide "grams". 100

was used for the bleaching of 45 feet of mm. color developed reversal film.

Photographic tests on sensitometric strips showed that the bleaching rate of the used or exhausted bleach solution dropped below the practical limit and that even extended bleaching of a small length of color developed 35 mm. reversal film failed to remove the residual silver stain.

The ferrocyanide ion concentration of the exhausted bleach solution was determined, according to the method of Brunner, Means and Zappert, described in the Journal of the Society of Motion Picture Engineers, vol. 53, page 30, and found to be 13.3 grams per liter.

To 1000 mls. of the foregoing exhausted bleach solution, 8.7 grams of sodium N-bromo-p-toluenesulfonamide and 11.8 mls. of 20% hydrobromic acid were added with stirring. After several minutes of stirring, the pH had been restored to its original value of 6.1. Photographic tests show that the bleaching action of the rejuvenated bleach was equal to that of the first solution.

Example II To 1800 mls. of used bleach of Example I and containing 2.12 grams of ferrocyanide ion per liter, 1.5. grams of N -dibromobenzenesulfonamide, 1.15 grams of potassium bromide and 1.15 grams of sodium bisulfate were added with stirring. After standing for 10 minutes, the solution had been completely regenerated, and showed excellent bleaching properties.

Example I'II To 2000 mls.. of used bleach, of Example I, and containing 5.27 grams of ferrocyanide ion per liter, 2.85 grams of N-bromoacetamide, and 8.4 grams vof 20% hydrobromic acid were added. Photographic tests showed that the performance of this regenerated bleach was equal to that of afresh bleach solution.

Example IV To 1500 mls.. of used bleach of Example I, and containing 5.27 grams of ferricyanide ion per liter, 2.52 grams of N-bromosuccinimide, 1.5 grams of sodium bromide and 1.4 grains of sulfamic acid. After stirring for several minutes, the solution was regenerated and showed excellent bleaching properties.

Example V To 1000 mls. of used bleach of Example I, and containing 6.35 grams of ferrocyanide ion per liter, 3 rams of N-bromobenzamide and 3.75

mls. of 20% sulfuric acid were added successively.

After standing for a few minutes, the pH had returned to its original values. Photographic tests showed the solution tohave satisfactory bleaching properties.

It will be noted that the procedures of Examples II and IV have the advantage inthat all of the regenerating chemicals are added in the for of solids. r

In the commercial processing of motion picture color film, the concentration of the various constituents of the bleach solution decreases gradually. Some water is brought into the bleaching tank by the wetted film and. small quantities of the ferricyanide salts are carried away by the bleached film. This dilution in most cases does not materially affect the bleaching rate, If desired, the. solution can, however, be kept at full strength by adding small amounts of the same constituents or chemicals originally used for the preparation of the bleach solution.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that I do not wish to limit myself to the specific embodiments thereof except as defined by the appended claims.

We claim:

1. In the photographic process of bleaching a metallic silver image to silver halide with a bleach solution comprising an alkali metal bromide and an alkali metal ferricyanide whereby the silver in the silver image is oxidized to silver ions which combine with the halide ions of the solution and form a silver halide in the emulsion and a rela tively small amount of the ferrocyanide, and then regenerating the exhausted bleaching solution by adding a regenerating agent, the improvement wherein said regenerating agent is a material selected from the group consisting of terminal primary ralkylamines, aliphatic and aromatic carbonamides, carbonimides, aromatic sul .f-onamides, and melamine, in which the respective amino, amido and imido nitrogen is substituted by at least one bromine atom, the amount of said N-bromo derivative being such as to release about one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleach solution.

*2. A photographic process according to claim 1, wherein there is added an amount of sodium'N- bromo p-toluenesulfonamide in a quantity sufiicient to release one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleach solution.

3. A--photographic process according to claim 1, wherein there is added an amount of dibfomobenzenesulionamide in a quantity sufiic ient torelease one molar equivalent of bromine for each molar" equivalent of ferrocyanide ion present in the exhausted bleach solution.

4. A photographicprocess according to claim 1, wherein there is added an amount of N -;bromoacetamide'in a quantity sufficient to releaseone molar equivalent of, bromine for each molar equivalent of. ferrocyanide ion present in the ex-v hausted bleach solution.

5. A photographic process according to claim 1, wherein thereis added an amount of N-bromosuccinimide in a quantity sufiicient to release one molar equivalent of bromine for each molar equivalent of ferrocyanide ion present in the exhausted bleachsolution.

'6. A photographic process according to claim 1,,

wherein there is added an amount of N-bromobenzamide in a quantity sufli-cient to release one molar equivalent of bromine for each molar equivalentof ferrocy-anide ion present in the exhausted bleach'solution.

7. In the photographic process of bleaching a metallic silver image to silver halide with a bleach solution comprising analkali metalbromide and an alkali metal ferricyanide whereby the silver in'the silver image is oxidized to silver ions which combine with the halide ions of the solution and form a silver halide in the emulsion and a relatively small amount of the ferrocy-anide, and then regenerating the exhausted bleaching. solution by adding a regenerating agent, the improvement wherein said regenerating agent is a materialse' lected from the group consisting of terminal primary alkylamines aliphatic and aromatic carbonamides, carbonimides, aromatic sulionamides,

'7, wherein there is added one-fourth mol of di-' bromo-benzenesulfonamide for each mol of ferrocyanide ion present in the I exhausted bleach solution.

'10. A photographicprocess according to claim' 7, wherein there is added one-half mol of Micromoacetamide for each mol of terrocyanide ion present in the exhausted bleach solution.

11. A photographic process according to'claim '7, wherein there is added one-half mol of N -bromosuccinami-de for each mol of ferro-cyanide ion present in the exhausted bleach solution.

.12. A photographic process according to claim 7, wherein there is added one-half mol oi N bromobenzamide for each mol of ferrocyani-de ion present in the exhausted bleach solution.

AUGUST H. BRUNNEFR, JR.

ROBERT H. ZAPPERT.

REFERENCES CITED Thefollowing references are of record in the file of this patent: V

UNITED STATES PATENTS Number Name Date 2,515,930 Scary July 18, 1950 OTHER REFERENCES Procedure for Rejuvenating Color Film Bleach Solutions, Ansco publication, received in Div. 6'7

on Oct.20, 1947, 11 pages, page 1 cited. 7

Whitmore, Frank 0., Organic Chemistry, Van

Nostrand Co. Inc, 1937, pages and 191 cited. 

1. IN THE PHOTOGRAPHIC PROCESS OF BLEACHING A METALLIC SILVER IMAGE TO SILVER HALIDE WITH A BLEACH SOLUTION COMPRISING AN ALKALI METAL BROMIDE AND AN ALKALI METAL FERRICYANIDE WHEREBY THE SILVER IN THE SILVER IMAGE IS OXIDIZED TO SILVER IONS WHICH COMBINE WITH THE HALIDE IONS OF THE SOLUTION AND FORM A SILVER HALIDE IN THE EMULSION AND A RELATIVELY SMALL AMOUNT OF THE FERROCYANIDE, AND THEN REGENERATING THE EXHAUSTED BLEACHING SOLUTION BY ADDING A REGENERATING AGENT THE IMPROVEMENT WHEREIN SAID REGENERATING AGENT IS A MATERIAL SELECTED FROM THE GROUP CONSISTING OF TERMINAL PRIMARY ALKYLAMINES, ALIPHATIC AND AROMATIC CARBONAMIDES, CARBONIMIDES, AROMATIC SULFONAMIDES, AND MELAMINE, IN WHICH TH RESPECTIVE AMINO, AMIDO AND IMIDO NITROGEN IS SUBSTITITED BY AT LEAST ONE BROMINE ATOM, THE AMOUNT OF SAID N-BROMO DERIVATIVE BEING SUCH AS TO RELEASE ABOUT ONE MOLAR EQUIVALENT OF BROMINE FOR EACH MOLAR EQUIVALENT OF FERROCYANIDE ION PRESENT IN IT EXHAUSTED BLEACH SOLUTION. 